1. Field of the Invention
The invention relates to devices suitable for providing a pathway to ground for static charges accumulated on substantially non-conducting surfaces including portions of the human body. More particularly the present invention provides a fastening structure combining a clasp and electrical connector for a simple static dissipative wristband, worn by personnel working close to static-sensitive electronic components and assemblies, to prevent damaged in the event of discharge of accumulated static charge.
2. Discussion of the Related Art
The known sensitivity of electronic circuits to accumulated static charge creates the need for precautionary measures to eliminate static discharge events that damage electronic components and assemblies. Such measures include configuring workstations with antistatic carpet, conductive or dissipative or grounded desktop work surfaces and ion generators used to neutralize static charges. Personnel involved in manufacturing integrated circuits and microelectronic components and assemblies use grounding straps to keep themselves and their surroundings at zero electrical potential. Grounding straps may comprise a fabric band wrapped around a wrist or ankle and secured in place by a clasp or buckle or similar mechanical fastener. Wrist straps and ankle straps include means for adjustment to assure a proper fit with the wearer""s limb. U.S. Pat. No. 5,184,274 illustrates several grounding strap concepts including features to ensure that the wearer""s skin is electrically connected to ground. This connection typically occurs by contact with a conductive surface on the inside of a strap. The conductive surface may include a discrete conductive element or, as described in U.S. Pat. No. 4,398,277 and U.S. Pat. No. 4,813,459, a layer or distribution of conductive fibers that contact the wearer""s skin. Some static-conductive wrist straps incorporate both a conductive element and conductive fibers for removing electrostatic charges via a suitable connection to ground. A connection to ground may include a separable snap connector to provide continuous electrical connection between a conductive element or conductive fibers and a wire (i.e. a ground cord) leading to ground. This arrangement of components directs accumulated static charge from a wearer to ground via the wrist strap and the ground cord. The ground cord typically has a built-in current limiting resistor to prevent electrical shocks to the wearer.
Personnel in electronics manufacturing industries have a broad array of grounding straps to choose from. Products range from expensive and durable, for multiple refuse, to low-cost grounding straps, such as that described in U.S. Pat. No. 4,698,724, which is intended for disposal after one or two uses. Expensive grounding straps comprise multiple parts including an insulating material portion having a resistance of more than 1014 ohms combined with a conductive portion with an electrical resistance of less than 105 ohms. Structures for securing wrist and ankle bands are usually separate from conductive connecting elements used for attaching a ground cord. U.S. Pat. No. 4,677,521, U.S. Pat. No. 4,847,729 and U.S. Pat. No. 5,196,985 provide examples of grounding straps of the type described.
Other devices that assist in dissipation of accumulated static charge include buckles and clasps that may be used with a variety of different types of strap material. U.S. Pat. No. 4,755,144 describes a static grounding buckle for securing a conductive strap. The buckle has a conductive base for skin contact and a non-conductive cover partially attached to the base. A conductive attachment element is on the non-conductive cover and a conductive clip member is within the non-conductive cover to provide electrical continuity between the conductive base and the conductive attachment element. While not attached to the strap, the buckle may be used to secure a strap to a user""s wrist or ankle by gripping the loose ends of the strap. A similar gripping device, described in CH 665,073, has two parallel metal tongues, produced by folding a metal strip. The upper tongue has a conductive stud protruding from its surface for connection of a ground wire. After wrapping a band around a limb of a user, the spring force of the tongues of the metal strip grip the loose ends of the band to secure it to the limb. This places the lower tongue of the metal strip in contact with the wrist or ankle of the user to provide a conducting path from skin to ground. Lacking connection to a band or strap, loose buckles or clasps are easily lost or misplaced. Also, buckles and clasps hold loose strap ends using clamps that are separate from connectors for ground wires.
Regardless of the large number of concepts for static dissipative products (resistance of between 105 ohms and 109) and antistatic products (a resistance of between 109 ohms and 1014 ohms) there remains a need for grounding static control straps satisfying short-term use requirements, which have a relatively low cost without the use of loose parts.
The present invention satisfies the need for inexpensive static charge dissipating straps meeting performance needs between expensive, extended use straps and low cost disposable straps. As described herein static control wrist straps and static control ankle straps have the benefits of low cost, being substantially injection moldable and fabricated from few parts. Preferably a static charge dissipating strap has two parts or less. It is conceivable that static control straps according to the present invention may be a single-piece integral structure.
The development of designs having fewer parts contributes to reduction of manufacturing costs associated with limited lifetime products. Parts reduction according to the present invention occurred because of the discovery that a single structure acts as a fastener to secure the wristband and at the same time provides a ground cord connector. Previously disclosed charge dissipating devices required separate structures for securing a wristband and providing connection to a ground cord. As indicated above, known wrist straps and ankle straps etc. differ from the present invention by showing the need for one part to secure a strap and a separate part to connect the strap to a ground plane. Another difference is the common combination of non-conductive materials, to provide strength to a wristband, with one or more conductive materials that provide a path to ground for accumulations of electrostatic charge.
More particularly the present invention provides an article for dissipating accumulated static charges from a substantially insulated surface. The article comprises a single layer elongate conductor, having opposing ends, and at least one fastener located to join the opposing ends together to provide a continuous conductive loop in contact with the substantially insulated surface. The fastener further provides electrical connection to a ground cord for removal of accumulated static charge from the substantially insulated surface. A fastener may be either electrically conductive or electrically insulating provided it facilitates contact between the elongate conductor and a conductive portion of a ground cord.
The present invention further provides a method for removing accumulated static charge from a substantially insulated surface. Steps for removing static charge include providing a static charge removal device that has a single layer elongate conductor having opposing ends, a ground cord connected to a ground plane and at least one fastener located for joining together the opposing ends of the elongate conductor. After shaping the elongate conductor to provide a continuous conductive loop in contact with the substantially insulated surface, and using the fastener to join the opposing ends of the elongate conductor, a connection involving the ground cord and the fastener directs accumulated static charge away from the substantially insulated surface to ground.
Terms used herein have the meanings indicated as follows:
A xe2x80x9csingle layer elongate conductorxe2x80x9d refers to a strip of conducting material, preferably a plastic material of uniform composition that includes electrically conductive particles selected from particles consisting of metal and conductive carbon.
Terms such as xe2x80x9cstatic dissipative strapxe2x80x9d or xe2x80x9cstatic control strapxe2x80x9d or xe2x80x9cstatic control bandxe2x80x9d or the like refer to single layer, elongate conductors placed in contact with substantially insulating surfaces to remove accumulation of static charges from the insulating surface. Such a strap or band may be further described in terms of a xe2x80x9cwrist bandxe2x80x9d or xe2x80x9cgrounding strapxe2x80x9d or xe2x80x9cankle strapxe2x80x9d and related terms.
The term xe2x80x9cground cordxe2x80x9d refers to an electrically conducting filament, typically including a metallic wire, having one end attached to ground and the other adapted for connection to a static control strap or band.
Terms including, xe2x80x9cfastener,xe2x80x9d or xe2x80x9cconnector,xe2x80x9d or xe2x80x9cclaspxe2x80x9d and the like may be used herein interchangeably to describe a dual function, single element used to clamp or clasp the ends of an elongate conductor or static dissipative strap while, at the same time, acting as a point of attachment for a ground cord. A dual function, single element is a distinguishing feature of the present invention preferably in the form of a rivet or stud that may be either electrically conducting or non-conducting.
The beneficial effects described above apply generally to the static control straps disclosed herein. The specific structures though which these benefits are delivered will be described in detail hereinbelow.